The present invention relates generally to corrosion and material damage detection; and more particularly to a nondestructive method and apparatus for optical detection and monitoring corrosion and material degradation in metal and composite structures which are generally inaccessible to light and visual inspection.
A heavy toll on material and maintenance costs on military as well as commercial aircraft can be attributed to the severity of the environment in which they operate. Under the influence of environmental effects or corrosion, susceptibility to stress-corrosion cracking and corrosion fatigue of critical aircraft structural materials (steels and aluminum) increase by a factor greater than ten and significantly reduce the useful life of aircraft. Even newer aircraft which use advanced materials such as graphite/epoxy composites are susceptible to such effects. Frequently, they are detected too late for any simple measure to be taken to repair damaged parts. Occasionally, if the corrosion or environmental effects were not discovered in time, the results could be catastrophic.
As current fleets of aircraft age without new aircraft entering a fleet inventory, the degrading effects of corrosion become more critical in terms of maintenance, readiness and safety. Flying aircraft near their expected useful life might actually be well beyond their safe life. Due to limited resources, some aircraft are not retired at their original expected lives but are reconditioned to fly beyond that time. Consequently, frequent inspections, preventive maintenance, and repairs require older aircraft to be removed periodically from service for costly and extended periods of time. In many cases, it is necessary to remove the aircraft's skin to access parts for inspection further adding to cost and down-time.
All of these considerations indicate that early detection and quantification of corrosion is extremely important, especially for carrier-based Navy aircraft which are exposed at sea to extremely corrosive environments.
Extensive studies in the area of corrosion detection and prevention have been carried out in the laboratory. In connection with these studies, electrochemical sensors (current) and optical (color) sensor for detecting early signs of corrosion have been investigated.
Electrochemical sensors are either incorporated in coatings or installed in a structure to produce signals when there is corrosion or damage, and before the effects become too severe. The sensing elements are bimetallic galvanic ultrathin-film devices fabricated on a polymeric film to generate a current when exposed to moisture.
Optical sensors, on the other hand, require reduction-oxidation (redox) chemicals which produce a change in an optical property such as a color or fluorescence when exposed to visible or ultraviolet light. However, the choice of inspection sites in structures for optically detecting corrosion is greatly limited because they must be accessible to both light and observation at the sensors. For instance, optical changes produced by redox reactions and/or corrosion in lap joints, under protective coatings or paint and on the backside of aircraft skin are particularly difficult to observe.